1. Field of the Invention
The present invention concerns an optical recording medium, and, more particularly, relates to a method of recording, reproducing and erasing information on and from an optical recording medium which uses a photochromic material for a recording film.
2. Description of the Related Art
A photochromic material is used in various fields, such as a light-intensity adjusting filter, a display device, a light intensity-measuring unit, a recording medium in a photoprinting system and dye etc..
Because of the possibility of its use as a reversible memory medium that ensures high-density recording, the photochromic material has recently been receiving attention in the field of optical recording media which use a laser beam.
Recording, reproduction and erasure to an optical recording medium which has a recording film made of such a photochromic material, particularly, bis(thienylethene anhydride)-type recording material, are generally performed in the following procedure. For instance, when recording light of a wavelength of 420 nm is irradiated on the recording film, a coloring reaction occurs on the irradiated portion, forming colored recorded pits there. When erasure light of a wavelength of 550 nm is irradiated on the recorded spots, the color of the colored recorded pits disappear, thus erasing the recorded pits.
If light of a wavelength of 420 nm is used to reproduce (read) information, however, the colored pit portion remain intact but the other uncolored portion therearound is colored. As a result, the whole area around the spot will be colored, disabling the distinction of the actual recorded portion from the unrecorded portion. In other words, the recorded information is erased. If light of a 550-nm wavelength is used to read information, on the contrary, the recorded pits are colored, erasing the information. The biggest bottleneck of an optical recording medium using a photochromic material is that no repetitive information reproduction (reading) is possible.
As a solution to this shortcoming, recently has been proposed a method of using mixed light of light with a 420-nm wavelength (hereinafter referred to simply as "420-nm light") and light with a 550-nm wavelength (hereinafter referred to simply as "550-nm light") to reproduce the recorded information. The principle of this method will now be briefly described below. (1) In reproducing a recorded portion, if mixed light of 420-nm light and 550-nm light is irradiated on the recorded portion, the recording material generates heat when absorbing the 550-nm light so that its color will disappear. This heat accelerates the color reaction by the 420-nm light, keeping the colored status. (2) If the mixed light of 420-nm light and 550-nm light is irradiated on an unrecorded portion, this portion does not absorb the 550-nm light, thus generating no heat. When the unrecorded portion absorbs the 420-nm light, a slight coloring reaction takes place and it will be canceled out by the 550-nm light. The unrecorded portion therefore remains unrecorded.
Even if this conventional method is employed, a slight color reaction occurs at the time of reproducing an unrecorded portion though it would not be recorded as a recorded portion. When reproduction is performed repeatedly, this slight coloring will trigger a change in temperature so that the temperature of the unrecorded portion will rise due to the absorption of the 550-nm light. Eventually, the unrecorded portion will be colored enough so that it is no longer distinguishable from the recorded portion.